CHARACTERISTICS OF FALLING STAGE SYSTEMS TRACTS FROM THE CARBONATE MARGIN OF THE TACONIC FORELAND BASIN: IMPLICATIONS FOR CRATONIC SEQUENCE STRATIGRAPHY

Forced regressive strata, deposited during relative sea-level fall, are common components of foreland-basin successions, although their documentation is still largely restricted to the siliciclastic margins of these two-sided systems. New data from mixed carbonate-siliciclastic Upper Ordovician and Lower Silurian strata of the Cincinnati Arch and northern Appalachian Basin provide detailed documentation of falling stage systems tracts (FSSTs) developed on the carbonate margin of the Taconic foreland basin in eastern North America.

These regressive components of fourth-order depositional sequences are dominated by widespread, heavily amalgamated skeletal sand sheets (argillaceous calcarenite). Although relatively thin, these deposits show many of the characteristics of their siliciclastic-dominated counterparts, including: (1) a sharp, erosional basal surface, (2) a position between the highstand systems tract and the sequence boundary, (3) a shallowing-upward pattern (in some cases through several fifth-order cycles), (4) evidence for long-distance regression, and (5) “foreshortened” stratigraphic successions. Lower Silurian FSSTs can be traced from the carbonate margin across the foredeep into progradational sandstone tongues that build out into the basin from the siliciclastic margin. All of these strata were deposited during “transitional periods” between icehouse and greenhouse climates, yet share many characteristics with those of Pennsylvanian Midcontinent cyclothems deposited during full icehouse conditions.

The great similarity between these FSST deposits from the middle Paleozoic strata of the Cincinnati Arch suggests a common response to relative sea-level fall. Further, the similarity of facies and stratal architecture of these deposits to those described by Tucker and others (1993) for the Triassic Muschelkalk carbonate platforms of eastern Spain suggests they could be integrated into a general model of cratonic sequence stratigraphy. A vital aspect of such a model would be to distinguish between the highly winnowed and amalgamated FSST calcarenites deposited under conditions of decreasing accommodation from overlying condensed and often mineralized transgressive skeletal grainstones and rudstones deposited under conditions of increasing accommodation.